This invention relates to a cathode ray tube that is substantially arc-free during normal operation. More particularly, this invention relates to an improved cathode ray tube having an internal electrically resistive coating comprised of a glass containing iron and TiO.sub.2, and an internal electrically conductive coating comprised of a glass containing iron.
A cathode ray tube is usually comprised of a funnel portion terminating at one end in a neck portion, and terminating at the other end with a faceplate portion. In the neck portion there is located a cathode comprised of an electron gun assembly. The funnel portion includes an anode, which is typically a metallic conductive anode button disposed in the wall of the funnel in the region adjacent to the area where the faceplate joins the funnel. The interior of the faceplate is coated with a fluorescent or phosphorescent material.
In cathode ray tubes used in television receivers, an aperture mask is usually interposed between the electron gun and faceplate. The aperture mask functions to produce electron beams, which activate the fluorescent and phosphorescent materials on the interior of the faceplate.
A cathode ray tube also has a conductive coating on the interior of the funnel portion and interior of the neck portion. The electrons projected by the electron gun assembly toward the interior of the faceplate are conducted away from the faceplate and other interior surfaces of the tube to the anode button by means of this conductive coating.
A product known as "Aquadag", and commonly referred to throughout the television industry as "dag", is generally used as the internal conductive coating. Dag is comprised of particulate graphite and an alkali metal silicate binder, and is usually applied in aqueous form to the interior of the funnel and neck portions of the cathode ray tube. An electrically conductive coating is formed after the dag coating is baked onto the surface at an elevated temperature.
Cathode ray tubes of the prior art are characterized by two types of internal arcing. The first type of arcing is attributable to characteristics of the dag coating. During normal tube processing and in subsequent handling, graphite particles and dag flakes are released from the dag coating. When these particles and flakes are struck by electrons from the cathode, arcing can occur within the cathode ray tube. Also, electrical leakage in the electron gun may occur. Furthermore, when the graphite particles are struck by the electrons from the gun, carbon gas may form. This results in gassing within the tube, and can also result in poisoning of barium in the electron gun. In cathode ray tubes in which a mask is interposed between the gun and faceplate, the particles and flakes can even be driven into the mask resulting in blocking of apertures in the mask. In some cases, the particles and flakes can be driven through the mask and into the fluorescent and phosphorescent materials on the faceplate. All of these phenomena shorten the life of the tube, and can adversely affect the operation of the receiver or instrument in which the tube is used.
Solutions to this problem have been proposed in U.S. Pat. No. 3,791,546 and U.S. Pat. No. 3,792,300. In each of these patents, however, dag or a modification thereof is still employed. Thus, a certain amount of flaking is to be expected.
The second type of arcing encountered in a cathode ray tube is attributable to the difference in electrical potential between the electron gun assembly and the interior surface of the neck portion of the tube in which the gun is located. It is believed that this type of arcing is dependent upon the degree of vacuum, the composition and topography of the electron gun parts and inner surface of the cathode ray tube opposite the gun, and the electric potential. This latter factor has assumed increasing importance in the color television industry where brighter pictures are being sought by increasing the operating voltage of the picture tube. This type of arcing is generally preceded by the appearance of a bluish island in the affected zone, usually between the electron gun and neck portion of the tube, and then the formation of a brilliant white flash within the tube.
This second type of arcing and solutions to it are described in greater detail in U.S. Pat. No. 3,355,617 and U.S. Pat. No. 3,758,802. U.S. Pat. No. 3,355,617 discloses a conducting coating of resistive material on the inner wall of the neck of the cathode ray tube. A highly resistant, electrically conductive film comprised of iron and manganese oxide is disposed on the walls of the neck of the tube adjacent the final anode of the electron gun assembly and coextensive therewith. A second conductive coating having less resistance than the film is joined to the film and is disposed around the neck of the tube adjacent to and coextensive with the electrodes of the gun, which are at a potential lower than the final anode. The film is electrically connected to the higher potential final anode and the coating is electrically connected to the lower potential electrodes so that the voltage drop between the film and coating and the electron gun assembly is essentially zero. The coating and film are applied in the form of water soluble salts of iron, magnesium and titanium. A baking operation is employed to drive off the water and decompose the salts to yield the oxides of the metals. This patent still discloses the use of Aquadag in the interior of the funnel portion and a forward portion of the neck of the tube. It can be expected that the internal coatings and films will be subject to flaking and peeling during assembly and subsequent processing.
U.S. Pat. No. 3,355,617 teaches that arcing between electrodes of a cathode ray tube can be minimized by providing a coating of crystallized glass on the inner surface of the neck and coextensive with the electron gun, together with a conductive material on the inner surface of the neck and spaced from the coating of crystallized glass. The patent teaches that the conductive layer can be graphite. Again, it can be expected that graphite particles will be released, thereby giving rise to the first type of arcing previously described.
In addition to being harmful to the cathode ray tube, arcing within the tube can cause severe damage to the electrical circuitry outside the tube because of the large electrical currents produced. This is particularly true when the receiver or instrument contains solid state components and devices in its circuitry. For example, the normal operating current at the anode button of a cathode ray tube in a color television receiver is about 20 - 80 milliamperes. Arcing within the tube can give rise to currents at the anode button as high as about 200 amperes. Such large currents can severly damage solid state componentry. The color television industry is now seeking to limit peak current at the anode button to about 30 amperes or less, and preferably about 10 amperes or less.
Accordingly, there exists a need in the art for a substantially arc-free cathode ray tube. The tube should be capable of being operated at high voltages without substantial internal arcing. The tube should be provided with an internal coating which is capable of serving the same function as the dag coating now used in prior art devices. The new coating should be resistant to abrasion, scratching, flaking and peeling in order to obviate the problems now encountered with dag coatings. Furthermore, the tube should be capable of being operated at test voltages of about 40,000 volts and operating voltages as high as about 35,000 volts substantially without the second type of arcing described herein. Additionally, it is essential that the tube be capable of manufacture according to the techniques commonly used in large scale manufacture of cathode ray tubes without requiring substantial changes in these techniques. The coating to be applied to the internal surfaces of the tube should be capable of being bonded to the tube during the firing cycles now employed. It is essential that the physical and electrical properties of the coating exhibit the required stability when the tube is fired and re-fired during assembly of the various components of the tube. When the cathode ray tube is employed in a color television receiver, the peak currents at the anode button should be about 30 amperes or less, preferably about 10 amperes or less, during normal operation of the receiver.